Research has been actively conducted for semiconductor pressure sensors using micromachining technology which is the micro-fabrication technology using the semiconductor fabricating process. Semiconductor pressure sensors which are micro-fabricated by micromachining technology are widely used in fields, such as automobile systems, industrial control, environmental monitoring, biomedical diagnosis, etc.
Based on pressure sensing principles, semiconductor pressure sensors used as devices to measure absolute pressure or relative pressure are classified into a metal pressure sensor in a strain gauge type, a piezoresistive pressure sensor, a piezoelectric pressure sensor, a MOSFET pressure sensor, a piezojunction pressure sensor, an optical pressure sensor and a capacitive pressure sensor. Among these pressure sensors, except for the metal pressure sensor they are manufactured by micromachining technology using silicon, a semiconductor substance.
In a capacitive pressure sensor, a parallel plate capacitor is positioned between a silicon thin film membrane and a support. This capacitive pressure sensor uses the principle that the capacitance value varies as a gap between two electrodes changes by the deflection of a silicon thin film diaphragm (that is, deformation of the membrane) according to pressure applied from the outside.
The above-described capacitive pressure sensor, which detects the capacitance value between two electrodes by using the deformation of the membrane, has several tens to several hundreds of times higher value (that is, higher sensitivity) in comparison with a piezoresistive pressure sensor. It also has excellent stability (that is, lower temperature coefficient and stronger structure) and low power consumption (that is, lower power use).
FIG. 1 is a cross-sectional view illustrating a conventional capacitive pressure sensor.
In FIG. 1, the conventional capacitive pressure sensor includes a cavity 20 formed on a substrate 10. The cavity 20 is formed on a silicon oxide film 14 formed on the substrate 10. A silicon oxide film 32 having openings 32a is formed over the cavity 20.
A low resistance layer 12 functioning as a lower electrode is formed within the substrate 10 under the silicon oxide film 14. A polycrystalline silicon film 34 functioning as an upper electrode is formed on the silicon oxide film so as to partially cover the silicon oxide film 32. The low resistance layer 12 and the polycrystalline silicon film 34 are electrically separated from each other and connected to exposed contact electrodes 50, 51.
A silicon oxide film 36 is formed on the polycrystalline silicon film 34 relatively thickly so as to bury the openings 32a of the silicon oxide film 32 and between patterns of the polycrystalline silicon film 34. A silicon nitride film 40 is formed on the silicon oxide film 36. The silicon nitride film 40 has the tensile stress property to compensate the compressive stress of the silicon oxide film 36.
However, in the aforementioned conventional capacitive pressure sensor, since the polycrystalline silicon film 34 functioning as the upper electrode is formed to expand to the silicon oxide film 14 over the cavity 20, a parallel plate capacitor (see “A” in FIG. 1) is formed between the polycrystalline silicon film 34 and the low resistance layer 12 functioning as a lower electrode. Accordingly, during the process of detecting the capacitance value according to the deformation of a membrane, the capacitance value by the parallel plate capacitor acts as noise, making it substantially impossible to detect micro-pressure.
Moreover, in the conventional capacitive pressure sensor, since the silicon oxide film 36 needs to be vapor-deposited relatively thickly in order to cover the polycrystalline silicon film 34 functioning as the upper electrode and to bury the openings 32a of the silicon oxide film 32 formed under the polycrystalline silicon film 34, there is a certain limit in realizing a micro-sensor for sensing micro-capacitance. In addition, since the silicon nitride film 40 needs to be additionally formed on the silicon oxide film 36 as “B” indicated in FIG. 1, the thickness increases in the direction where pressure is applied and therefore it is more difficult to realize a micro-sensor.